A primary factor affecting channel re-use in a cellular, or other radio communication system is the carrier to interference (C/I) ratio or more specifically the energy per bit to total spectral noise ratio (E.sub.b /N.sub.0) in a digital system such as TDMA. In providing more channels in any given structure, such as a cell site, you necessarily decrease the C/I ratio. One contributing factor to the C/I ratio is the amount of energy radiated by a cell site. Therefore, if you can control the amount of energy that is radiated by the cell site, you can have a net-positive impact on the C/I ratio.
It shall be understood that increasing the C/I ratio, is desirable as it indicates a larger signal strength on your carrier signal, which is the phone call or other communication that is being served by a particular cell site, and a smaller signal of interferer, such as a foreign user with respect to a particular cell site. Therefore, it is desirable for the energy density of the signal that is being served by a particular cell site to be as great as possible with respect to the mobile units being served by a particular cell.
In the past, attempts to improve the C/I ratio at any given structure have included adjusting the output power of the cell footprint so that there is minimal overlap between adjacent cells. However, this method is limited in its ability to reduce interference because, without the ability to dynamically alter power levels, elimination of the overlap entirely is not possible if a sufficient signal is to be available at the cell's fringe.
Likewise, where there is an obstruction between a communication device and the central structure, such as a cellular base transceiver station (BTS), provision of a sufficient signal to this communication device may involve a signal, in areas not affected by the obstruction, sufficient to have an undesirable overlap of adjacent cells. Therefore, a fixed power level, adjusted to provide adequate service in the shadow of an obstruction, may very well lead to a decreased C/I ratio at adjacent cells.
This problem is compounded by the fact that even when having adjustable power to maintain a minimum overlap, simply adding more channels to a particular cell site to accommodate more users at that cell site, increases the energy density. Therefore, the interference level at adjacent cells, is increased proportionately to the number of added channels because more communication devices at an adjacent cell are transmitting at a fixed power level sufficiently high to overlap the service area.
Therefore, there is a need in the art to minimize the forward transmitted power level by adjacent cells to provide a signal sufficient to sustain communication throughout a predefined area, while avoiding any overlap of an adjacent predefined area when possible. As a signal sufficient to sustain communication at the boundary of two adjacent predefined areas or outboard of an obstruction is likely to also be of a sufficient strength to overlap this boundary, it is desirable to be able to dynamically adjust the power level so as not to overlap the boundary when no communication device is operating at or near the boundary.
As communication systems often use time division multiple access signals, there is also a need in the art to provide a time division signal with independent power level adjustment of the various time divisions of the signal to avoid overlap in each time division where no communication device is operating at or near the boundary or outboard of an obstruction.
There is further need in the art for a system to dynamically provide a power adjusted signal to individual directional antenna elements of a plurality of antenna elements to reduce signal overlap to only those areas where a communication device is operating.